There are various technologies explored for targeting sound and particularly audible sound to be heard at particular region(s) in space (i.e. bright zones) while being suppressed at other regions (i.e. dark zones) such that in those regions the sound pressure level is below the hearing threshold or is sufficiently low such that it is perceived as part of the surrounding noise.
Existing solutions for generation of targeted sound can roughly be classified into two main technological categories:                Technologies utilizing the conventional acoustical wave theory for manipulating audible sound waves (i.e. sound waves of relatively long wavelengths).        Technologies utilizing the so called non-linear air-borne ultrasound modulation for generation of audible sound. These techniques manipulate the frequency content of non-audible ultrasonic (US) waves (i.e. sound waves of relatively short wavelengths) and rely on the non-linearity of the sound propagation medium (e.g. air/water) for the generation of audible sound from the short ultrasonic waves.        
Technologies utilizing the conventional acoustical wave theory for manipulating long audible waves are disclosed for example in U.S. Pat. No. 5,532,438. Products utilizing such technologies include for example the Secret Sound® directional speaker system product of Museum Tools and the focused arrays product of Dakota Audio (e.g. the floor mounted focused arrays product FA-603).
The phenomena of air (and water) non-linear medium behavior under high SPL sound wave transmission was discovered 45 years ago when experimenting on sonar waves for submarines (see “Parametric Acoustic Array” by Peter J. Westervelt, published in The Journal of the Acoustical Society of America” volume 35, number 4, April 1963, pages 535-537). This effect is described mathematically by the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation which describes the propagation of waves in space in consideration of waves interference, waves dispersion and non-linear response of the medium (e.g. air) through which the waves propagate. An approximation typically used for solving the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation on the depth axis (axial direction) is provided for example in “Possible exploitation of non-linear acoustics in underwater transmitting applications” by H. O. Berktay, published in J. Sound Vib. (1965) 2 (4), 435-461.
Technologies utilizing the non-linear air-borne ultrasound modulated technique can generally be categorized to two main approaches, each providing a somewhat different result, and each suited for different purposes. According to one of these approaches, a directional audio beam demodulates from high frequency ultrasound waves at high sound pressure level (SPL). This approach generally provides the transmission of a highly directional and relatively narrow audio beam propagating along a predetermined direction with low decay rate in the SPL along this direction. Systems operating in accordance with this approach include for example Audio Spotlight™ by Holosonic Research labs, inc., HSS—hyper sonic sound system by Audionation-Uk Ltd (e.g. HSS model 3000) and also products of LRAD Corporation.
An alternative approach for utilizing the non-linear air-borne ultrasound modulated effect is based on focusing ultrasonic wave beams to a predetermined region. Technologies based on this approach are disclosed for example in U.S. Pat. No. 6,556,687 and in U.S. Pat. No. 7,146,011. This technology, however, did not mature to commercial device implementation due to difficulties in providing appropriate focusing capabilities.